Salinity
Salinity means consisting of or containing salt. In the case of the ocean, it means the dissolved salts that have in through rivers. When river water flows into the ocean, it carries in all of the salty minerals it has picked up. Ocean water may evaporate, but the salt does not. This causes the rest of the ocean to get saltier and saltier. The term salinity explains the relative concentration of salt. A salinometer is used to measure the saltiness in the ocean. If the ocean is not salty enough in some places, the water cannot get dense enough to sink. Since warm water can hold more NaCl and other molecules than colder water, warm, beach areas are likely to be saltier than colder water. This means that colder water can rest upon warmer water if the warm water has high salinity, and that the natural flow of a current can be reversed based on the related density, salinity, and temperature of the water.
Density
Density is the quality of being dense. Ocean water is denser than fresh water because of all the salt in it. The deeper in the ocean you go, the denser the water gets because of pressure. Temperature also affects density. The colder the water, the denser the water. Less dense water, like the ocean water in the continental shelf, floats on more dense water, such as water in the twilight zone. You can think of ocean water as a triple layer cake: dark chocolate (most dense water) on the bottom, milk chocolate (around twilight zone, middle density) in the middle, and white chocolate (shallow water, very little density) on the top. Both density and salinity affect deep sea currents. Changes in density and salinity cause the deep sea currents. Not much marine life lives in very dense water near the ocean floor.